Not Applicable
1. Field of the Invention
The present invention relates to method for manufacturing a hybrid golf club shaft. More specifically, the present invention relates to a method of manufacturing a hybrid golf club shaft composed of a thermoplastic material and a thermosetting material.
2. Description of the Related Art
With the advent of composite golf club shafts, it has become much easier to tailor the design of a golf club to the needs of a particular player or particular shot. For example, for longer shots and lower numbered irons, or woods, it is often desirable to use a more flexible shaft. Whereas, for shorter shots and higher numbered irons, it is often more desirable to use a stiffer shaft. Such design goals may be achieved, for example, through the use of additional layers of composite fiber in shorter shafted clubs and through the use of fewer layers of fiber in longer dubs. Such design goals may also be achieved by varying the orientation of the layers of composite fiber that make up a shaft. For example, to add stiffness to a club shaft it may be desirable to utilize several layers of composite fiber that run parallel to the longitudinal axis of the shaft, whereas to enhance the flexibility of a shaft it may be desirable to utilize several layers of composite fiber which are offset to a substantial degree, for example, +/xe2x88x9245xc2x0 or more, from the longitudinal axis.
With regard to the manufacture of composite golf club shafts, those skilled in the art will appreciate that each layer of composite fiber may be formed using a xe2x80x9cpre-pregxe2x80x9d composite sheet, and that pre-preg composite sheets may be manufactured by pulling strands of fiber, for example, carbon or glass fiber, through a resin solution and allowing the resin to partially cure. Exemplary resins or xe2x80x9cbinding matricesxe2x80x9d may include, for example, thermoset epoxy resins and thermoplastic resins. Alternatively, prexcx9cpreg sheets may be manufactured by pulling a fabric or weave of composite fabric through a resin solution and allowing the resin to partially cure. In either case, once the resin is partially cured or xe2x80x9cstaged,xe2x80x9d the resin holds the fibers together such that the fibers form a malleable sheet.
It also will be appreciated that the performance characteristics of a golf club shaft may be significantly altered through the choice of resin or binding matrix used within the shaft. For example, it will be appreciated that composite materials bound within a thermoset binding matrix may be used to produce stiffer shafts, whereas similar composite materials bound within a thermoplastic resin may produce a softer feeling shaft. Further, it will be noted that shafts manufactured from thermoset materials are generally harder and more brittle than shafts manufactured from thermoplastic materials, and that thermoplastic materials provide a greater amount of vibration damping than do thermoset materials when used within a golf club shaft. Those skilled in the art also will appreciate that better impact tolerance characteristics may be achieved through the use of thermoplastic materials as opposed to thermoset materials.
Prior to this time, however, those skilled in the art have encountered substantial difficulty in developing a hybrid golf club shaft that provides the benefits associated with both thermoplastic and thermoset resin pre-pregs. One reason for this is that, prior to the conception of the subject invention, difficulty was encountered in providing a durable bond between the layers of thermoplastic and thermoset resin within a golf club shaft. Moreover, when attempts were made to produce a hybrid thermoplastic/thermoset golf club shaft, substantial issues of durability were encountered as a result of sheering between the respective layers. Stated somewhat differently, when attempts were made to produce hybrid thermoplastic/thermoset golf club shafts using conventional methodologies, the resulting shafts were not able to pass standard durability requirements. For example, when fitted with club heads and swung by a mechanical ball-hitting machine, the shafts were unable to withstand 550 high heel hits without incurring substantial delamination of the resin layers comprising the shafts.
In view of the foregoing, it is submitted that those skilled in the art would find an improved hybrid thermoplastic/thermoset composite golf club shaft capable of passing rigorous durability testing, and methods of manufacturing such shafts, to be quite useful.
The present invention is directed to methods of manufacturing hybrid golf club shafts composed of a thermoplastic and a thermoset material.
In one particularly innovative aspect, the present invention is directed to a method for manufacturing golf club shaft comprising one or more plies or layers of composite fiber bound within a thermoset resin and at least one ply or layer of composite fiber bound within a thermoplastic resin, wherein the layer of fiber bound within the thermoplastic resin is bonded via a co-curing process to a layer of fiber bound within the thermoset resin by a high peel strength nylon epoxy film adhesive. It is believed that, by virtue of the use of the high peel strength nylon epoxy film adhesive within golf club shafts in accordance with the present invention, those shafts are able to better endure conventional durability testing procedures and, indeed, that those shafts are able to endure even the most rugged durability testing procedures. Moreover, it is believed that the high peel strength nylon epoxy adhesive film creates a durable bond between the dissimilar pre-preg materials.
Thus, it will be appreciated by those skilled in the art that a shaft manufactured in accordance with the present invention may be quite durable while exhibiting the preferred characteristics of both thermoplastic and thermoset products. For example, it will be appreciated that because fibers bound within a thermoset resin may comprise the core of a shaft in accordance with the present invention, such shafts may exhibit the same torsional performance characteristics as conventional shafts. Further, through the utilization of one or more layers of fiber bound within a thermoplastic resin around the periphery of the shafts, shafts made in accordance with the present invention may exhibit improved damage resistance characteristics and, perhaps of equal importance, will provide a softer feel due to the vibration damping characteristics of the thermoplastic resin.
Those skilled in the art also will appreciate that the subject invention has applicability to both filament winding and pre-preg layering processes. For example, to manufacture a golf club shaft in accordance with the present invention using a pre-preg wrapping process, the following steps are preferably followed. The dimensions and relative positions of the plies of thermoset pre-preg and thermoplastic pre-preg composite fiber sheet are determined, and a set of plies to be used in the shaft is prepared. A ply of bonding adhesive, preferably comprising a high peel strength nylon epoxy film adhesive, is also prepared. The plies of thermoplastic pre-preg are then wrapped around a mandrel in a predetermined manner and shrink-wrapped and cured according to a prescribed curing cycle. Thereafter, the shrinkswrap is removed and a ply of adhesive is wrapped over the pre-cured thermoplastic plies. Then, one or more plies of thermoset pre-preg are wrapped over the adhesive. The entire assembly may then be shrink-wrapped and cured. Following the curing process, the shrink-wrap is removed from the assembly, and the outer layer of the shaft is sanded and finished to specifications.
In an alternative embodiment, a bladder molding process may be used. In such a process, a mandrel having predefined dimensions is selected and covered by a bladder manufactured, for example, from silicone. Plies of thermoplastic pre-preg are then wound around the bladder-covered mandrel in a predetermined manner, placed in a mold, and pre-cured. The mandrel and bladder then are removed from the pre-cured part. Thereafter, a second bladder-covered mandrel is prepared, one or more plies of thermoset pre-preg are wrapped around the second bladder-covered mandrel, and a ply of adhesive is wrapped over the plies of thermoset prepreg. The thermoset and adhesive wrapped mandrel is then inserted into a cavity formed within the pre-cured thermoplastic part, and the entire assembly is placed in a mold. A source of pressurized gas may then be used to inflate the bladder and force the plies of thermoset pre-preg against an inner wall of the pre-cured thermoplastic part, and the mold may be placed in an oven for a selected period of time, i.e., a time sufficient to allow proper curing of the resin comprising the various thermoset and thermoplastic plies. Thereafter, the second mandrel and bladder may be removed from the core of the shaft, and the shaft itself may be removed from the mold.
In one alternative embodiment, the plies including a thermoplastic resin may be wrapped around the mandrel and pre-cured in a mold using a bladder process and/or a shrink-wrap process. After the pre-cure, the adhesive and ply (or plies) including the thermoset resin may be wrapped over the pre-cured layers of thermoplastic pre-preg. Following that, a cellophane or polypropylene tape may be wrapped over the layer(s) of pre-preg including the thermoset resin, and the pre-preg wrapped mandrel assembly may be placed within a mold. The part may then be heated to a predetermined temperature for a time sufficient to allow the layers of pre-preg and adhesive to fully cure.
In alternative embodiments, a filament winding process may be used to wrap the layers of fiber over the mandrel and any intervening plies. However, in such embodiments it is preferable to apply a ply of adhesive between any fiber layers that are bound within differing binding matrices (i.e., thermoset and thermoplastic matrices).
Having briefly described the present invention, the above and further objects, features and advantages thereof will be recognized by those skilled in the pertinent art from the following detailed description of the invention when taken in conjunction with the accompanying drawings.